This invention relates to a method for controlling the oxygen content of agglomerated molybdenum-containing powders by controlled reduction thereof. More particularly it relates to a method of introducing a controlled amount of oxygen into agglomerated molybdenum-containing powders by reducing powders of molybdenum dioxide and one or more ammonium-containing compounds of molybdenum.
Flame spraying and plasma spraying are common techniques for the application of protective and wear resistant coatings of various metals, ceramics, and cermets, usually to metal surfaces (substrates). Flame spraying is commonly used to produce molybdenum-containing coatings on piston rings for internal combustion automobile engines and synchronizer rings for manual transmissions for automobiles and trucks.
An electric arc or flame can be used to melt the end of a continuously fed coil of molybdenum wire as a gas propels the molten droplets which form onto a substrate for example, the wear surface of a cast iron piston ring where it splats and solidifies, forming the coating in successive layers. Because of the presence of excess oxygen either from the flame or the surrounding air, or both, the coatings produced by this technique contain large quantities of oxygen, typically from about 7% to about 8% in solution and as various molybdenum oxides. The large quantities of oxygen in the molybdenum harden the coating.
In the plasma spraying of molybdenum, there is usually a minimum of oxygen in the sprayed coating due to the use of an oxygen-lean plasma gas system. That is, argon, helium, hydrogen, nitrogen, or combinations of these gases, all of which are relatively free from oxygen, are used in the plasma spraying process. Hence, any oxygen in the sprayed coating is due to oxidation of the molten particles caused by oxygen from the surrounding air mixing with the plasma gas and/or surface oxidation of the freshly deposited coating. In such "pure" molybdenum coatings the oxygen level is in the 1% to 2% range. Such coatings are softer than their flame sprayed counterparts.
For higher hardness, therefore, a more expensive process such as the flame spray process which requires wire, or a more expensive powder such as molybdenum plus nickel-base alloy must be used.
It would be desirable therefore to have a method of producing molybdneum powders of sufficiently high oxygen content to enable them to be used in a plasma spray process to produce hard coatings.
U.S. Pat. No. 4,624,700 relates to a method to control the oxygen content of agglomerated molybdenum powders by controlled reduction of the powders. This is done by reducing a mixture of molybdenum and one or more oxygen containing compounds of molybdenum under controlled conditions. The mixture is made up of molybdenum and typically mixtures of compounds such as molybdenum dioxide, molybdenum trioxide and ammonium paramolybdate.
U.S. Pat. No. 4,146,400 relates to a method for introducing a controlled level of oxygen into an agglomerated molybdenum metal powder involving heating the powder at a sufficient temperature for a sufficient time in the presence of water vapor and a non-oxidizing atmosphere with the amount of non-oxidizing atmosphere being controlled to produce a partially-oxidized molybdenum powder.
U.S. Pat. No. 4,146,388 describes and claims molybdenum plasma spray powders and a process for producing the powders of molybdenum and oxides of molybdenum having an oxygen content of from about 0.5 to about 15% by weight oxygen. The process involves passing molybdenum particles through a plasma with oxygen or oxides of molybdenum to produce the oxidized powder.